Mono-Inyo Craters is a north–south-trending
volcanic chain in Eastern California that stretches
25 miles (40 km) from south of Mammoth
Mountain to the northwest shore of Mono Lake. Politically, it is located in Mono County in the U.S.
State of California. Mono Lake Volcanic Field
forms the northernmost part of the chain and consists of two
volcanic islands in the lake and one cinder cone volcano on its northwest shore.
Most of the Mono Craters, which make up the bulk
of the northern part of the Mono-Inyo chain, are phreatic
(steam explosion) volcanoes that have since been either plugged or
over-topped by rhyolite
domes and lava flows. Inyo Craters form much of
the southern part of that line and are either phreatic explosion
pits or rhyolitic lava flows and domes. The southernmost part of
the chain consists of fumaroles and explosion pits on Mammoth
Mountain and of a set of cinder cones south of the mountain; the
latter set is called the Red Cones.

The area has been exploited by humans for centuries. Obsidian was collected by Mono Paiutes for making
sharp tools and arrow points, and was used as a trade object.
Glassy rock continues to be removed from the volcanoes in modern
times for use as commercial scour and yard decoration. Mono
Mills processed timber felled on or near the volcanoes for the
nearby boomtown Bodie in the late 19th to early 20th
centuries. Water diversions into the Los
Angeles Aqueduct system from their natural outlets in Mono Lake
started in 1941 after a water tunnel was cut under Mono Craters.
Mono Lake Volcanic Field and a large part of Mono Craters gained
some protection under Mono Basin National Forest Scenic Area in
1984. The Los Angeles
Department of Water and Power accepted a state decision in 1994
to cut its water diversions from Mono Lake. Resource use along all
of the chain is managed by the United States Forest
Service as part of Inyo National Forest. Various
activities are available along the chain, including hiking, bird
watching, canoeing, skiing, and mountain biking.

Eruptions along the narrow fissure system that underlies the
chain began in the west moat of Long Valley Caldera 400,000 to
60,000 years ago. Mammoth Mountain was formed during this period.
Multiple eruptions from 40,000 to 600 years ago created Mono
Craters and eruptions 5,000 to 500 years ago formed Inyo Craters.
Lava flows 5,000 years ago built the Red Cones, and explosion pits
on Mammoth Mountain were excavated in the last 1,000 years. Uplift
of Paoha Island in
Mono Lake about 250 years ago is the most recent activity. These
eruptions most likely originated from small magma bodies rather
than from a single, large magma chamber like the one that produced
the massive Long Valley Caldera eruption 760,000 years ago. During
the past 3,000 years, eruptions have occurred every 700 to 250
years. In 1980, a series of earthquakes and uplift within and south
of Long Valley Caldera indicated renewed activity along the
chain.

Mono Craters is a 10.5 miles (16.9 km) chain of at
least 27 volcanic domes, three large glass flows called coulees and various explosion
pits and other associated volcanic features.[9]
The domes of the chain lie on a roughly north–south-trending arc
that is concave to the west and located south of Mono Lake. The
highest of the Mono Domes is Crater Mountain (elevation
9,172 ft or 2,796 m), which rises 2,400 feet
(730 m) above Pumice Valley to the west .[10]
Associated volcanic features are located in Mono Lake (Paoha and Negit Islands) and on
its north shore (Black Point). The coulees cluster north and south
of the overlapping chain of domes. Craters exist at the top of most
domes and on flat land south of them.

Inyo Craters are a set of explosion pits and flows of glassy
lava.

The two southernmost Inyo Craters are open pits in a forested
area that are about 600 feet (180 m) across and 100 to
200 feet (30 to 61 m) deep, each with small ponds covering
their floors.[11]
A quarter mile (half kilometer) north of these is another Inyo
explosion pit on top of Deer Mountain. Farther north of these
craters are five lava
domes, including Deadman Creek Dome, Glass Creek Dome, Obsidian
Dome, and Wilson Butte. These domes are composed of gray rhyolite, frothy pumice, and black obsidian. The Mono-Inyo
Craters volcanic chain extends into Long Valley Caldera but is not
related to the caldera's volcanism.[10]

South of Inyo Craters proper are other features related to the
dike system
responsible for creating the craters, volcanoes and lava flows.
These include a north–south trend of fault scarps up to 20 feet
(6.1 m) high and pull-apart cracks or fissures in the
earth.[12]
These fissures can not technically be called faults because
little or no vertical or horizontal movement has occurred along
them.[13]
Most notable among these is "Earthquake Fault", a fissure up to
10 feet (3.0 m) wide that cuts 60 to 70 feet (18 to
21 m) into glassy rhyolite lava flows.[12]
Stairs to the bottom of the fissure were removed after being
damaged by earthquakes in 1980.[12]
Several Mono-Inyo-related explosion pits are on Mammoth
Mountain.[2]
The Red Cones, south of Mammoth Mountain, are basaltic cinder cones and are
the southernmost part of the Mono-Inyo Craters volcanic chain.[14][2]

Climate and
ecology

Mule Deer are common in the area

The desert environment of Mono Basin receives about
14 inches (36 cm) of precipitation a year.[15]
Annual precipitation around Mammoth Lakes, which is close to the
Inyo Craters, is about 23 inches (58 cm).[16]
Moisture travels over the Sierra crest from the Pacific Ocean
through the San Joaquin Gap.[17]
Temperatures in Mono Basin range from average winter lows of 20 to
28 °F (-7 to -2 °C) to average summer highs of 75 to
84 °F (24 to 29 °C).[15]
Temperatures near the Inyo Craters and Mammoth Lakes area range
from winter average lows of 16 to 21 °F (-9 to -6 °C) to
summer average highs of 70 to 78 °F (21 to 26 °C).[16]

Most of the surface of the Mono Craters is barren but slopes on
the sides of that part of the Mono-Inyo range is covered by Jeffrey Pine
forest.[18]
Pumice Valley, directly to the west, is covered by sagebrushscrubland.[18]
The soil consists primarily of deep pumice, which does not hold
water well.[17]Mycorrhizae fungi in the soil invade the
roots of Jeffrey Pine trees in a symbiotic relationship that helps
the pine absorb water and provides nutrients to the fungi.[17]
Jeffrey Pine forests also surround the Inyo Craters[19] and
Mammoth Mountain.[20]Mule
Deer, Coyote, Black
Bear, Yellow-bellied Marmot, Raccoon and Mountain Lions all have ranges that are
coincident with forests that cover parts of the Mono-Inyo
craters.[21]

Typical
evolution

Aerial photo of Panum Crater

Panum Crater is
the northernmost volcano in the sequence and is a good example of
both a tuff ring and a rhyolite dome. Its
structure is two-fold; an outer tuff ring (forming a classic
crater) and an inner plug, or dome of rhyolite, pumice and obsidian
created from lavas.[22]
In this case, the magma feeding Panum reached the surface, as lava
after its heat had already created a steam explosion crater. Other
Mono Craters also were formed in this manner, but their plug domes
grew larger than their tuff ring craters. The domes have steep
sides and are flanked by slopes of scree consisting of large angular and glass-rich
rocks.[23]
Devil's Punch Bowl, located south of the main dome complex, stopped
forming at an earlier stage of development. It is a 1,200-foot
(370 m) wide and 140-foot (43 m) deep explosion pit with
a much smaller glass dome on its floor.[23]

Northwest Coulee

The large North and South Coulee and the smaller Northwest
Coulee consist of obsidian-rich rhyolite. They were formed from
slow-moving pasty lava that had a thin and brittle crust. Once the
flow stopped, it formed steep sided tongues of sharp and angular
rock that are typically 200 to 300 feet (61 to 91 m) thick and
have scree piles along their
base.[24]
South Coulee is 2.25 miles (3.62 km) long,
0.75 miles (1.21 km) wide and has a volume of
0.1 cubic miles (0.4 km3); making it the
largest Mono coulee in volume.[25] South
Coulee originates from the crest of Mono Domes, about 3 miles
(4.8 km) from the southern end, flows down its east and west
flanks and terminates at its foot.[24]
North Coulee is nearly as large, flows mostly to the east and
terminates in a divided pair of lobes. Northwest Coulee is located
northwest of North Coulee and was intruded by Upper Dome after the
coulee solidified.[26]
Permanent pockets of ice from snowmelt have been found 75 to 147
feet (23 to 45 m) inside the coulees and domes.[26]

History

Exploitation

Obsidian from the
Mono-Inyo Craters was collected by native Mono Paiutes for making sharp tools and
arrow points, and was used as a trade object.[27]
Unworked obsidian was carried by the Mono Paiutes over passes in
the Sierra Nevada to trade with other Native American
groups. Chips of Mono-Inyo obsidian can still be found at many
ancient mountain campsites.[28]

Mark Twain visited
Mono Basin in the 1860s and wrote about Mono Lake, but did not
mention any of the Mono-Inyo Craters except for the lake's two
volcanic islands. He wrote in Roughing It that the lake was in a
"lifeless, hideous desert..." that was the "loneliest spot on
earth... little graced with the picturesque."[29] This
book was published in 1872 and helped shape attitudes about the
area.

Mono Mills processed wood near Mono Domes for use in the boom town
of Bodie.

Gold rush-related boomtowns sprang up near and
in Mono Basin in the 19th century to exploit local bonanzas. The
largest of these, Bodie (north of Mono Lake), was
founded in the late 1870s and was successful enough to need a tree
mill. The tree mill was located at Mono
Mills, immediately northeast of Mono Domes.[30] Trees
directly around the domes and on their slopes were felled to
provide timber for the mill.

Falling water levels in Mono Lake have exposed large areas of
lakebed.

The purpose of the tunnel was to divert water from Rush, Lee
Vinning, Parker and Walker creeks from their natural outlet in Mono
Lake into the Owens
River, which in turn was diverted farther downstream into the
Los
Angeles Aqueduct system.[31]
Once the tunnel was in use, the water level of Mono Lake dropped 1
to 1.6 feet (0.30 to 0.49 m) a year.[31]
The aqueduct system was expanded in 1970 and diversion from streams
feeding Mono Lake intensified.[33] Lake
level was at 6,417 feet (1,956 m) above sea level before
diversions began through the Mono Craters Tunnel and reached a low
of 6,372.3 feet (1,942.3 m) in 1981.[34]

The United States Pumice Company, based in Chatsworth,
California, has mined the area for pumice since 1941.[35] The
company markets the pumice in slabs for use in commercial scouring
and in large irregular chunks sold as yard decoration.[23]

Exploratory drilling for geothermal power occurred near Mono
Craters on the south shore of Mono Lake in 1971.[36] The
wells did not show promising results, so the effort was
abandoned.

Protection and
monitoring

Mono Basin National Scenic Area Visitor Center

The creation of Mono Basin National Forest Scenic Area in 1984
stopped any further geothermal prospecting.[37] Mono
Basin was the first National Forest Scenic Area in the United
States.[38]
It offers more protection than other United States Forest
Service lands, surrounds Mono Lake and its two volcanic
islands, Black Point, Panum Crater and much of the northern half of
Mono Craters.[39]

Litigation and outreach by the Mono Lake Committee, the National Audubon Society and
other conservation groups has helped to slow and, more recently,
reverse some water diversions from tributaries feeding Mono Lake
through the Mono Craters Tunnel. Starting in 1989, no diversions
were allowed if the lake level fell below 6,377 feet
(1,944 m) above see level, and in 1994 the Los Angeles
Department of Water and Power accepted a decision by the state
government to cut back its water diversions until the lake level
reaches 6,392 feet (1,948 m).[40]
In 1994, lake level was 6,374.6 feet (1,943.0 m) above
sea level and by 2008 it had risen 7.7 feet (2.3 m).[34]

A series of earthquakes inside Long Valley Caldera, coincidently
starting two weeks after the May 1980 eruption of Mount
St. Helens in Washington, alerted geologists to the possibility
of renewed volcanic activity in the region.[41][42][note 2]
Four magnitude 6 earthquakes
struck the southern margin of Long Valley Caldera in an area that
was close to the Mono-Inyo fissure system.[43]
The caldera floor had also uplifted by 10 inches (30 cm)
in five years.[41]
Upward movement of magma under the caldera was thought to be the
cause of the earthquakes and uplift.[41][note
3]

Monitoring equipment near Mono Mills.

Persistent earthquake swarms in 1982 prompted the
U.S. Geological Survey (USGS) to issue a
"potential volcanic hazard" notice for Long Valley.[45] That
same year, permanent monitoring of the area by the Long Valley Observatory
started.[43]
The hazard notice was lifted in 1984 after USGS scientists
concluded that magma had been injected into fissures below Mammoth
Mountain but had congealed underground.[46]
From 1990 to 1996, 150 acres (61 ha) of trees were killed
on Mammoth Mountain by 20% to 95% concentrations of carbon dioxide
(CO2) in the soil (less than 1% is normal).[47]
Chemical analysis of the CO2 indicated it was derived
from magma.[46]

Geology

Precursor
activity

The basement rock under the Mono-Inyo chain consists of the same
granitic and metamorphic
rock that the Sierra Nevada is made of. Above that are basaltic to rhyolitic volcanic rocks that are 3.5 million
to less than 760,000 years old.[10]
Volcanism occurred north of the chain, in the Bodie Hills, as far
back as 28 million years.[48]
In fact, nearly all the rock east of the Sierra Nevada is volcanic
in origin.[49]

The Bishop Tuff is a thick sequence of welded ash that forms much
of the basement rock of the Mono-Inyo chain.

Near what is now Long Valley, volcanoes erupted from 3.6 to 2.3
million years ago.[50]
Eruptions occurred in and around Glass Mountain in the same
area from 2.1 to 0.8 million years ago.[51] The
massive eruption of Long Valley Caldera some 760,000
years ago laid down a thick sequence of Bishop Tuff over the region.

The Mono-Inyo Craters system began with the eruption of basalt and andesite in the west moat of Long Valley
Caldera 400,000 to 60,000 years ago.[8]
Eruptions around 300,000 years ago filled the west moat with
800 feet (240 m) of basaltic lava.[52]
Basaltic and andesitic eruptive activity then moved to Mono Basin
and lasted from 40,000 to 13,000 years ago.[8]

Seismic data indicates that a magma chamber with an estimated volume of
50 to 140 cubic miles (200 to 600 km3) exists 5 to
6 miles (8 to 10 km) directly below Mono Craters.[53]
About 600 feet (183 m) of subsidence has occurred along a
ring fracture system above the chamber in the last 700,000
years.[53]
Mono Craters sit atop a 7.5-mile (12 km)-long arc on the
11-mile (18 km)-wide ring-fracture system.[53]
Magma feeding the domes may have exploited arc-shaped fissures
around an intrusion of granitic rock deep below the chain.[9]
This magma chamber is separate from the magma chamber under Long
Valley Caldera. The Mono Arc and Mono Craters may represent an
early stage of caldera development.[54][53]

Repeated eruption of dacite
and rhyodacite from
vents on the southwest rim of the caldera from 220,000 to 50,000
years ago formed Mammoth Mountain, a volcano composed of
overlapping lava
domes.[55]
Dacite and rhyodacite also were erupted in the Mono Basin from
100,000 to 6,000 years ago.[8]

Mono Craters,
Negit Island and Black Point

Eruptions in the last 5000 years along the Mono-Inyo chain

The Mono Craters were formed by multiple eruptions of silica-rich
rhyolite from 40,000 to 600 years ago.[8]
Black Point, today on the north shore of Mono Lake, is a flattened
volcanic cone of basaltic tephra that formed under the surface of a much
deeper Mono Lake about 13,300 years ago, during the most-recent glacial
period.[56]
Negit Island, in Mono Lake, was formed by several eruptive episodes
from 1600 to 270 years ago (as of 2000).[57]

None of the Mono Craters near the lake show the effects of wave
erosion but a hill at the southern end does show incision by a
beach line. This indicates that a high stand of Mono Lake reached
this area before the formation of the northern Mono Craters.[58]
Glaciers did not reach as far as the Mono Craters but
stream-rounded stones are found under and on the volcanoes. Stones
on the volcanoes were lifted from the ground as the volcanoes
grew.[59]

The most recent eruptive episode on the Mono Craters occurred
sometime between 1325 and 1365.[60]
A vertical sheet-like mass of magma, called a dike, caused groundwater to
explosively flash to steam, creating a line of vents 4 miles
(6.4 km) long.[60]
A mix of ash and pulverized rock, called tephra, covered about
3,000 square miles (7,770 km2) of the Mono
Lake region to a depth of 8 inches (20 cm) 20 miles
(32 km) downwind to 2 inches (5.1 cm) 50 miles
(80 km) downwind. (Wind direction varied during that
time).[60]

Pyroclastic
flows of glowing-hot clouds of gas, ash and pulverized lava
erupted from the vents in narrow tongues that extended up to
5 miles (8.0 km) away and covered 38 square miles
(98 km2) in the second phase.[61]
Second-phase magma had been largely depleted of gas and steam. The
rhyolite magma was rich in the mineral silica and thus oozed out of
the vents to form several steep-sided domes, including Panum Dome
and the much larger North Coulee flow.[62]
The youngest domes and coulees are 600 to 700 years old.[10]

Inyo Craters and Paoha
Island

The Inyo Craters were formed by eruptions of silica-poor
rhyolite 5,000 to 500 years ago.[8]
The most recent activity occurred just a few years after the 1325
and 1365 Mono Crater eruptions described above and was caused by a
dike of similar composition.[63]
The dike spread both horizontally and upward, eventually extending
6.8 miles (11 km) long and up to 33 feet (10 m)
wide.[64]
Ground above the dike was significantly cracked and faulted.

The town of Mammoth Lakes sits on top of ashbeds from the Mono-Inyo
eruptions.

A series of explosive eruptions later occurred at three separate
vents.[64]
Pieces of molten and solid rock were ejected, small craters were
formed, and a tall eruption column rose above the vents. Pumice and
ash covered an extensive area downwind and about 1 inch
(2.5 cm) of tephra was deposited where the town of Mammoth Lakes, California,
now sits.[63]
A pyroclastic flow traveled about 3.7 miles (6.0 km) from
the South Deadman vent.[64]
Some of the open pits filled with pasty lava to form South Deadman
Creek, Glass Creek and Obsidian Flow domes. Others, such as the
Inyo Crater Lakes near Deer Mountain, remained open and were later
partially filled with water. Smaller explosion pits on the north
side of Mammoth Mountain were also formed at this time.[54]
In the last 6,000 years, approximately 0.19 cubic miles
(0.79 km3) of magma has been erupted from the Inyo
part of the chain.[65]

The last recorded volcanic activity in the chain was at Mono
Lake between the years 1720 and 1850.[63]
An intrusion of magma below the lake pushed lakebed sediments up to
form Paoha Island.
Columns of rhyolite stuck through the sediment on the north part of
the island, and lava erupted on the northeastern corner to form a
group of seven dacite cinder
cones and a lava flow.[63]
Steam rose in columns hundreds of feet high (tens of meters) from
Hot Spring Cove on the island and the spring water was 150 °F
(66 °C) when geologist Israel Russell visited the island in the
early 1880s.[66]

Volcanic
hazards

The creation of Paoha Island 250 years ago was the most recent
activity along the chain.

The Long Valley to Mono Lake region is one of three areas in
California that are studied under the United States Geological
Survey's (USGS) volcanic hazards program (the other two areas are
the Mount Lassen area
and Mount
Shasta).[67]
These areas were chosen to be in the program because they have been
active in the last 2000 years and have the ability to produce
explosive eruptions in the future.[67]

The Mono-Inyo Craters have erupted at intervals of 250 to 700
years during the past 3,000 years.[8]
The most recent eruptions are from Panum Crater and the Inyo
Craters 500 to 600 years ago, and Paoha Island about 250 years
ago.[8]
Seismic soundings of the crust and the composition of lava indicate
that these eruptions most likely originated from small, discrete
magma bodies rather than from a single, large magma chamber like
the one that produced the Long Valley Caldera eruption 760,000
years ago.[8]
The rate of eruption over the last 1,000 years has increased, with
at least 12 eruptions occurring.[60]

Potential thickness of tephra
on the ground from eruptions of less than 0.25 cubic miles
(1.0 km3)

Each episode of volcanic activity in the past 5,000 years from
the Mono-Inyo Craters has expelled less than 0.25 cubic miles
(1.0 km3) of magma.[68]
The next eruption in the area will most likely be similar in size
to the small to moderate eruptions in the past 5,000 years.[68]

Tephra may accumulate to a thickness of 33 feet (10 m)
near an erupting Mono-Inyo vent.[68]
Areas downwind could be covered by more than 8 inches
(20 cm) of tephra at a distance of 22 miles (35 km)
and by 2 inches (5 cm) at 53 miles (85 km).[68]
Winds in the area tend to blow toward an east or northeasterly
direction more than 50 percent of the time, and toward any easterly
direction more than 80 percent of the time.[68]
In general, the grain size and thickness of ash gradually decrease
with increasing distance from a vent. Air routes east of the vent
are likely to get contaminated with volcanic ash.[68]

Areas as far as 9.3 miles (15 km) from an explosive
eruption could be covered by pyroclastic flows and surges;[68]
this coverage depends on the location of the vent or vents,
topography, and volume of magma erupted. Pyroclastic flows
generated from vents on Mammoth Mountain could travel farther than
9.3 miles (15 km) because of the extra momentum gained
from descending thousands of feet (hundreds of meters).[68]

Pyroclastic flows from Mammoth Mountain or other steep-sided
volcano will travel farther than flows originating from shorter
features.

Basalt lava typically forms thin flows that may reach more than
30 miles (50 km) from their vent.[68]
Dacite and rhyolite lavas typically produce short, thick flows that
seldom move as far as 3 miles (5 km) from their vent.[68]
These short flows often build mound-shaped features called lava
domes. Rock fragments thrown from a growing lava dome may reach 3
to 6 miles (5 to 10 km) from the dome.[68]
A partial collapse of the steep-sided growing dome can send
pyroclastic flows outward at least 3 miles (5 km).[68]
Taller domes tend to form larger pyroclastic flows that travel
farther.

Activities

The Mono Basin National Scenic Area visitor center is located
near Mono Lake just off U.S. Route 395. A
bookstore, an information desk staffed by U.S. Forest Service
Rangers and museum exhibits help to orient visitors.[38]
The Mono
Lake Committee has a staffed office and visitor information
center in Lee Vining on the corner of U.S.
Route 395 and 3rd Street.[69]
Information on camping, hiking, guided and self-guided tours can
all be obtained at either location.

U.S. Route 395 near Mono Domes

U.S. 395 is a scenic route that roughly parallels the
Mono-Inyo Craters volcanic chain. California State Route 120
provides access to the northern and eastern parts of the Mono
Domes, including Panum
Crater.[70][71]
Mammoth Scenic Loop (also called Dry Creek Road), provides access
to the Inyo Crater lakes and some of Inyo domes.[72]

The town of Mammoth Lakes, located near
the southern end of the chain and Mammoth Mountain, is the largest
populated area nearby.[73]Mammoth Mountain Ski Area is
located nearby and gondola
rides can be taken year-round (weather permitting) to the
mountain's summit.[74]
The summit of Mammoth Mountain provides panoramic views of the
craters and domes of the Mono-Inyo volcanic chain, Mono Lake, the
Sierra Nevada and Long Valley Caldera.

Mono Lake itself has
its own set of activities, including walking tours among towers of
tufa, boat tours of the lake, and
birdwatcher opportunities.[75] The
lake is too salty to support any fish but fishing is possible in
streams that feed Mono Lake. Hiking around and on the craters and
domes and mountain biking outside of the Scenic Area boundaries are
also available activities.

Mono Craters from U.S. 395

Notes

^
Geographically, Mammoth Mountain
(11,059 feet/3,371 meters) is considered part of the Sierra
Nevada mountain range, not the Mono-Inyo Craters mountain
range, but volcanically, Mammoth Mountain and the southern part of
Mono-Inyo Craters share the same precursor activity. Explosion pits
and fumaroles on the
mountain formed in the last 1,000 years are considered part of
volcanic activity directly related to the Mono-Inyo Craters.[2][3]